A standard such valve assembly has a normally stationary plate formed with a single output port and at least one inlet port opening adjacent each other on the planar upper face of this stationary plate. A movable plate has a planar lower face pressed against the plate and formed with a recess that forms a downwardly open pocket opening at this lower face. An actuating element, normally a lever, can slide the movable plate on the fixed plate to a flow position in which the pocket is aligned with one or both of the inlet ports and with the outlet port for flow therebetween and a blocking position not covering the outlet port at all so that there is no flow between the ports. In the flow position the extent of overlap of the pocket with the various ports can control the amount of flow and the ratio of any mix obtained. Such valves are extremely popular, but have the considerable disadvantage that the reversal of direction effected in the pocket of the movable plate causes turbulence that generates noise.
Hence German patent No. 2,356,326 of A. Fowell et al proposes fitting a screen inside the pocket. This screen extends parallel to the interface plane between the plates, spaced down from the floor of the pocket. Thus the water entering from the inlet port or ports flows up through this screen which has the standard noise damping effect. At the same time the screen does interfere with flow so that it creates noise, albeit of a different type, itself, and acts as a restriction. Furthermore the considerable flow through it can quickly clog it with particles, thereby requiring dissassembly of the valve to restore full and quiet flow.
Another system is suggested in German patent document No. 2,819,065 of H. Zwink. It has a screen on one of the side walls of the pocket. Such a system has only a modest sound-damping effect and also clogs with particles over time.